1. Field of the Invention
The present invention relates to an antenna module, and more particularly, to an antenna module including an antenna configured to transmit/receive a high-frequency signal.
2. Description of the Related Art
An example of an antenna module is a strip line cable integrated with an antenna (hereinafter also referred to simply as a strip line cable). A specific example of such a strip line cable is disclosed in Japanese Unexamined Patent Application Publication No. 8-242117. FIG. 6 is a perspective view illustrating an external appearance of a strip line cable 500 disclosed in Japanese Unexamined Patent Application Publication No. 8-242117.
As shown in FIG. 6, the strip line cable 500 includes insulators 510 and 512, a center conductor 514, conductors 516 and 518, and an impedance matching circuit 520. The strip line cable 500 has three regions including an antenna part 502, a transmission line part 504, and a counterpoise part 506.
The insulators 510 and 512 are formed of a flexible material. The conductor 516 is disposed on a lower surface of the insulator 510. The conductor 518 is disposed on an upper surface of the insulator 512. The center conductor 514 is a line-shaped conductor disposed on an upper surface of the insulator 510 such that the center conductor 514 extends in a longitudinal direction of the insulator 510. The insulator 510 and the insulator 512 are bonded together such that the upper surface of the insulator 510 is in contact with the lower surface of the insulator 512.
Note that the insulator 510 and the insulator 512 are not bonded together over their entire regions, but an end region with a length equal to approximately one-fourth the wavelength of the high-frequency signal of each insulator is not bonded. Hereafter, this end region where the insulator 510 and the insulator 512 are not bonded together is also referred to simply as the end region. More specifically, the end region of the insulator 512 extends in a direction perpendicular to the insulator 510. In the end region of the insulator 510, the insulator 510, the center conductor 514, and the conductor 516, in this end region, form the antenna part 502 such that a high-frequency signal is transmitted or received from or by the center conductor 514 in the antenna part 502. The insulator 512 and the conductor 518 in the end region form the counterpoise part 506.
In the region other than the end region, the insulators 510 and 512, the center conductor 514, the conductors 516 and 518, and the impedance matching circuit 520 form the transmission line part 504. In the transmission line part 504, the center conductor 514 and the conductors 516 and 518 form a strip line.
The impedance matching circuit 520 is disposed in the middle of the center conductor 514. The impedance matching circuit 520 has a line width greater than the line width of the center conductor 514. The impedance matching circuit 520 formed in this manner provides impedance matching between the antenna part 502 and the strip line of the transmission line part 504.
The strip line cable 500 disclosed in Japanese Unexamined Patent Application Publication No. 8-242117 has the following problem. That is, it is difficult to design the strip line cable 500 such that the transmission line part 504 is capable of being bent with a small radius while maintaining stability of the characteristic impedance without resulting in an increase in DC resistance. The strip line cable 500 may be used, for example, in a portable telephone. In recent years, the size of the portable telephone has become increasingly small. As a result, the space for installing the strip line cable 500 in the portable telephone has increasingly become narrow. Thus, there is a need for a transmission line part capable of being bent with as small a radius as possible. A method for satisfying the above need may be, for example, to reduce the thickness of the insulators 510 and 512 to reduce the rigidity of the strip line cable 500, which makes it possible to bend the transmission line part 504 with a small radius.
However, in the strip line cable 500, the reduction in the thickness of the insulators 510 and 512 results in a reduction in the distance between the center conductor 514 and the conductors 516 and 518, which leads to an increase in capacitance between the center conductor 514 and the conductors 516 and 518. This causes a shift in the characteristic impedance (for example, 50 ohms or 75 ohms) of the strip line of the transmission line part 504. To prevent the shift in the characteristic impedance, it is necessary to reduce the line width of the center conductor 514 to reduce the capacitance between the center conductor 514 and the conductors 516 and 518. However, the reduction in line width of the center conductor 514 results in an increase in DC resistance of the strip line cable 500. As described above, in the technique disclosed in Japanese Unexamined Patent Application Publication No. 8-242117, it is difficult to design the strip line cable 500 such that the transmission line part 504 is capable of being bent with a small radius while maintaining stability of the characteristic impedance and without resulting in an increase in the DC resistance.